I. Field
The subject technology relates generally to communications systems and methods, and more particularly to systems and methods that perform timing corrections that are applied to channel estimates across pilot symbols in wireless networks.
II. Background
Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. These channels are sometimes called subbands or subcarriers. The technology was first conceived during research into minimizing interference among channels near each other in frequency. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Generally, priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream.
In one area, OFDM has also been used in European digital audio broadcast services. The technology lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks. Orthogonal Frequency Division Multiplexing can be considered an FDM modulation technique for transmitting large amounts of digital data over a radio wave where OFDM operates by splitting a radio signal into multiple smaller sub-signals or sub-carriers that are then transmitted simultaneously at different frequencies to the receiver. One advantage of OFDM technology is that it reduces the amount of crosstalk in signal transmissions where current specifications such as 802.11a WLAN, 802.16 and WiMAX technologies employ various OFDM aspects. Another example of OFDM based wireless system is FLO (Forward Link Only). FLO is a wireless system that has been developed to efficiently broadcast real time audio and video signals to mobile receivers using the OFDM technology.
Wireless communication systems such as FLO are designed to work in a mobile environment where the channel characteristics in terms of the number of channel taps with significant energy, path gains and the path delays are expected to vary quite significantly over a period of time. In an OFDM system, the timing synchronization block in the receiver responds to changes in the channel profile by selecting the OFDM symbol boundary appropriately to maximize the energy captured in the FFT window. When such timing corrections take place, it is important that the channel estimation algorithm takes the timing corrections into account while computing the channel estimate to be used for demodulating a given OFDM symbol. In some implementations, the channel estimate is also used to determine timing adjustment to the symbol boundary that needs to be applied to future symbols, thus resulting in a subtle interplay between timing corrections that have already been introduced and the timing corrections that will be determined for the future symbols. Further, it is common for channel estimation block to process pilot observations from multiple OFDM symbols in order to result in a channel estimate that has better noise averaging and also resolves longer channel delay spreads. When pilot observations from multiple OFDM symbols are processed together to generate channel estimate, it is important that the underlying OFDM symbols are aligned with respect to the symbol timing. Without such alignment, erroneous channel estimates will be generated and thus proper operation of wireless receivers cannot be ensured.